The insulation design of high voltage apparatus and stations is determined by the switching impulse withstand levels and the creepage distance requirements, among other parameters. The increment of high voltage transmission levels has produced the rise of the switching withstand requirements. In order to reduce electrode surfaces stresses and to improve the voltage withstand capabilities, electrodes with large curvature radii are widely used in high voltage apparatus, bus terminals and interconnections. The electrodes should provide a point free of corona and also fulfil the breakdown requirements according to the level of voltage of the station. The most important electrical characteristics of an electrode in dielectric gas from the insulation point of view are inception of corona discharges and breakdown voltage characteristics. Corona discharges are self sustainable partial discharges close to the highly stressed electrode. For indoor condition, the shape of the electrode and the length of clearance should be so determined that corona discharge will not occur under maximum operation voltage. The inception voltage of corona discharges is determined by the air gap geometry and atmospheric conditions, such as density and humidity.
Further development of the corona discharge with the increasing of the applied voltage may lead to the breakdown of air clearance. For external insulation design the shape of the electrode and the length of the clearance should be so determined that the breakdown of the clearance will not occur under over-voltage. A breakdown of air gap will start from corona inception and then the further development of stronger discharge activities.
Spherical electrodes are used e.g. in high voltage direct current (HVDC) valve halls for interconnecting various electrical equipment such as busbars, transformers and valves, busbars and bushings, and support insulators. The diameter of such spherical electrodes depends on the maximum voltage to be handled at an electrode interconnection point determined by the insulation coordination requirements, i.e. requirements of the electric strength of equipment in relation to the voltages which can appear on the system for which the equipment is intended. The aim of such a spherical electrode is to provide a more lenient electric field at the interconnection point and to prevent electrical stress such as corona discharges and breakdown of the air clearance.
Calculation of the clearance distances for electric equipment in the valve hall are based on switching impulse withstand voltage requirements, i.e. an overvoltage applied which an insulation media should be able to withstand at switching impulse operation. The clearance distance depends on the so called gap factor provided by the electrode (relation of the breakdown voltage of the arrangement at certain distance with the breakdown voltage of a point-plane arrangement at the same gap distance) and the breakdown media (insulators or air). If the electrode can provide a high switching impulse withstand voltage, the clearance distance that will fulfil the insulation requirements will be smaller than the clearance distance calculated with an electrode of a lower switching impulse withstand level capabilities.
The insulation distances of the electric equipment to ground plane are mainly formed by a combination of post insulators and top electrode and/or a combination post insulators, pedestal insulator and top electrode. Hence, the spherical/corona electrode is typically arranged on top of one or more vertically arranged post insulators. Possibly, the post insulators are arranged on top of a pedestal insulator arranged on the floor of the valve hall. Field tests using spherical electrodes have shown that to fulfil insulation requirements for indoor conditions at high voltages like 800 kV DC, the length of the post insulators should be 8 meters or larger and the pedestal insulator used should be at least 2.4 meters, implying that the spherical electrode is arranged at a height of 10.4 meters in the valve hall.
As HVDC operational voltages have been increased, the insulation requirements are increasingly harsher. To be able to meet the insulation requirements, the diameter of the corona free electrodes (e.g. spherical electrodes) has been increased. For interconnections in valve hall, the spherical electrodes for initial projects used 1.3 meters diameters, and currently 800 kV DC projects use spherical electrodes of 1.8 meters diameter or more. With the increase of the insulation requirements, the electrode diameter and the length of the post insulators used in the station have been increased.
However, the increase in electrode diameter is neither directly proportional to the increase in gap factor, nor to the increase in withstand voltage of the electrode. In addition, the relation breakdown voltage versus total length of pedestal and post insulators is not linear, and therefore an increase in post insulator length will not directly increase the breakdown voltage capabilities of the set-up. Experimental and theoretical research has shown that the relation between breakdown voltage and gap distance for spherical electrodes has a tendency to saturate, i.e. it does not matter in practice at what height the high voltage electrode is arranged, the breakdown voltage of the electrode will not increase over a certain height. This saturation point is however difficult to predict and the physical behaviour of the breakdown for this kind of set-up is complex and thus difficult to model.
Research has shown for an 1100 kV DC system spherical electrode with a diameter of 2 meters and insulation distance of 15 meters or more, the minimum 50% switching impulse withstand voltage required can not be fulfilled. Additionally, saturation of the gap is reached and higher voltages can not be reached by increasing insulation length, distance to floor or electrode diameter.